Bone cements based on acrylate plastics and bone replacement materials prepared therefrom have been known for a long time. Polymer materials based on acrylic and/or methacrylic acid esters have proved suitable therefor because of their biocompatibility, their outstanding strength properties, their favorable properties regarding release of embedded pharmaceutical active compounds and, last but not least, because of their processability appropriate for their use.
The usual bone cements are composed of a solid component, which comprises a finely divided polymer of acrylic and/or methacrylic acid esters and further additives, such as polymerization catalysts and, if appropriate, X-ray contrast media, fillers and dyestuffs, and a liquid component, which comprises an acrylic and/or methacrylic acid ester monomer and further additives, such as polymerization accelerators and stabilizers. For use, the solid component and liquid component are mixed to give a liquid to semi-solid paste, and this is brought into a desired shape, if appropriate, or applied to the implantation site for cementing in a prosthesis. The composition is hardened completely by the polymerization reaction induced when the components are mixed. The bone cement is expediently provided in a form in which separate containers with amounts of the two components which are coordinated with one another are combined as a pack unit. As a general rule, the proportion of solid component is about 50 to 75% by weight and the proportion of liquid component is about 50 to 25% by weight.
A bone cement which, in a normal pack, comprises two sachets of about 40 g of polymer powder each and 2 ampoules of 20 ml of monomer liquid each, for example, is very common. The powder is a fine bead polymer of methyl methacrylate with a copolymer content of methyl acrylate. About 0.5% of dibenzoyl peroxide is added to the powder as a catalyst. Small amounts of chlorophyll are also copolymerized during preparation for identification of the material. The powder additionally comprises a customary X-ray contrast medium, such as, for example, zirconium dioxide. The associated liquid comprises monomeric methyl methacrylate, to which about 0.7% of dimethyl-p-toluidine is added as a polymerization accelerator and small amounts of hydroquinone are added as a stabilizer. This liquid is also usually colored with a small amount of chlorophyll for identification. The powder, which is packed in polyethylene sachets, is sterilized with ethylene oxide. The liquid is subjected to sterile filtration and dispensed into glass ampoules.
When 2 parts by weight of powder are mixed together with one part by weight of liquid, dibenzoyl peroxide reacts with the dimethyl-p-toluidine in the liquid, and free radical polymerization is initiated by this means. The mixture is coordinated such that it can be used as a doughy paste after only about one minute. This paste remains kneadable for several minutes and then starts to harden, with evolution of heat. After about 5 to 10 minutes, the polymerization has essentially ended. During the polymerization phase, as long as the paste can still be shaped, it can be brought into any desired shape, that is to say, for example, can be introduced directly into the body for filling bone cavities or for cementing in prostheses, or can be used for the production of shaped articles which harden extra-corporeally and can then be used at any desired positions in the body.
While the clinical results with such bone cements are chiefly very good with implantation of endoprostheses, the prosthesis as a general rule being surrounded only by a uniform thin cement sheath which provides the bond between the prosthesis and the bone bed, problems as far as clinical failure often arise if relatively large amounts of cement in thick layers are necessary because of the implantation conditions or the field of use. This is the case, for example, if relatively large bone defects which must be filled with bone cement are present when a prosthesis is changed or after resection of bone tumors. One reason for the problems which occur lies in the exothermic polymerization reaction during hardening of the bone cement. Significant increases in temperature occur in cement thicknesses above about 4 mm, since the heat of reaction developed can no longer be distributed and removed adequately. For example, a temperature of about 100.degree. C. can easily be reached inside a cylindrical shaped article of bone cement of about 3 cm diameter during hardening. Heat necroses in the bone bed or in tissue surrounding the implantation site are the consequence.
Another problem factor is the shrinkage of bone cement based on acrylate, which is of more consequence the thicker the cement layer. This causes damage to the implant bed, which can lead to premature loosening right up to breakage of the prosthesis.
The strongest possible bond with the original bone or its fragments is the aim in the case of implantation of endoprostheses and also in the case of implantable shaped articles for bone replacement in the context of osteosynthesis. This can be achieved effectively only with intimate meshing, extending ideally to complete growth of regenerated bone matrix throughout the implant material. Nevertheless, a precondition of this is an adequate porosity, ideally with an interconnecting pore system, of the bone replacement material.
Bone replacement materials having a porous and, where appropriate, also interconnecting pore structure with a high mechanical stability at the same time are known. However, these are essentially ceramic shaped articles which are obtained by sintering, for example, calcium phosphate materials, such as hydroxyapatite or tricalcium phosphate, or by pyrolysis and sintering of natural bone. With these materials, it is of course possible only to fill bone defects.
A porous implant material with an interconnecting pore system based on calcium phosphate ceramic particles and bioabsorbable polymer is known from EP 0 519 293 A1. This material is also suitable only for filling bone defects, and because of its low mechanical strength is unsuitable for replacement of high-load bone structures. Although this material is plastically deformable to a certain extent, it is not suitable for anchoring endoprostheses in the sense of bone cement.
The lowest possible porosity is aimed for in customary bone cements for reasons of the mechanical strength required of the prosthesis-bone cement-bone bed bond. For this reason, the bone cement components are preferably mixed in vacuo with subsequent compression, so that inclusions of air and the resulting pore formation are avoided here as far as possible. To improve the long-term bond with the bone bed, it is advantageous to add osteoconductive additives to the bone cement. Possible such additives are chiefly finely divided calcium phosphate materials, such as hydroxyapatite and tricalcium phosphate, which are more or less bioabsorbable. Such bone cements which can comprise up to 35% by weight of such calcium phosphates having a particle size of up to 300 .mu.m are known from EP 0 016 906 and EP 0 148 253. However, these particles are for the most part embedded in the polymer material of the bone cement and enclosed by this. A certain porosity into which the bone matrix can grow can therefore only develop in the course of healing of the cemented prosthesis or of the bone cement implant into the surface regions of the bone cement in contact with the bone bed by absorption of calcium phosphate particles on the surface.